I have been purchasing some dubia online from a well known website. The problem is that when the dubia went through their last molt, it turned out that a couple of them were actually discoid roaches. Now I have about half Dubias and Discoids in my roach coach...lol. Is it okay to house them together or should I start seperating them?

No they can't climb smooth surfaces. At least, I haven't seen them. I could understand how the company I bought them from could mix them up. Before they become adults, they look exactly like dubia. I just hope they won't have any territory conflicts with the dubia and start eating them.

You will have no problems.
In fact there is a roach breeder here in Colorado
who sells dubia/discoid hybrids.
At some point you will probably end up with these as well.
These roaches are very compatible.

I kept these two together without a problem. Easy to distinguish the two.

Most probably they were sending you samples, not a mistake. They probably just forgot to tell you. Like Brad R., many people really get into the whole insect breeding. They are really interesting, and it is easy to get carried away with multiple species. (Don't worry Brad, I too am in the same position as you. Where/when will it stop, I just don't know...)

Manga actually likes these better at times just because they are a little more active and fiesty. I didn't have any problem with any flying or climbing (but I also had vasaline around the rim of the tub.)